Wiring new detached garage/studio
I'm in the process of building a detached garage/painting studio, and I have a few general questions about wiring it. I've checked out a stack of books on the subject from the library, but I want to go over a few things before I start. I will pull the electrical permits in the next few days.
The garage is 14' x 24'; the studio is 10' x 17', sharing an interior wall with the garage. The interior walls of both the studio and garage will be finished with drywall. This site shows the overall design:
My plan is to put a 12/24 subpanel in the garage with a 60 amp main breaker for 5 circuits. I buried a 1" PVC conduit 18" under ground, coming up through the foundation stem wall. The other end comes up about 7" from our basement wall (an obstruction prevented me from getting any closer.) I have a string going through the pipe. (When I buried the pipe last year I didn't know I should use a long sweep turn to transition from the vertical to horizontal pipe; I hope that won't be a problem.) I had the concrete guys stub up a long (like 14') piece of rebar up through the foundation wall to ground the panel to.
I'm planning to run four #6 THHN conductors to the subpanel, 2 hot (red, black), 1 neutral (white), 1 ground. Once in the basement I'd run the conductors through metal conduit to a 60 amp breaker in our main panel, which was upgraded by licensed electricians to 200 amps last year in anticipation of this garage project. I don't know if I'm required to have a main breaker in the subpanel in the garage, but I'd like one out there just to be on the safe side.
In the garage subpanel I'd make sure the neutral and ground bus aren't bonded together.
A question about "load centers" as they're called at Home Depot. I've seen what looks like essentially the same model, except one is called Main Lug and one is called Main Circuit Breaker. What's the difference, and which would I use?
I'm planning to run 5 circuits for the structure, all 20 amp using 12-2 NM-B cable:
In the studio:
One circuit for 6-7 outlets, a switched lighted ceiling fan and switched exterior light.
One circuit for an outlet dedicated to a window air conditioner.
One 240 volt circuit for an in-wall space heater with fan.
In the garage:
One circuit for 5 outlets on the south side, a switched exterior light, and 4 overhead switched hanging fluorescent lighting fixtures (the standard 4' pairs of tubes).
One circuit for 5 outlets on the north side, an exterior outlet and the garage door motor.
I believe I need GFCI protection in the garage circuits. Would I just use GFCI outlets upstream (closest to the subpanel) for each of the two outlet circuits, or should I use GFCI breakers in the subpanel?
I'll also be running some low-voltage wiring for the garage door sensors and switch. I'm drilling separate holes in the studs, top plates and rafters for that wiring. I'm not planning to run more than two NM-B conductors through one 3/4" hole.
That's about it. Please let me know if my approach seems sound or if you see any pronounced inefficiencies.
Thank you for your expertise.
Also, what kind of 90º fitting did you use? I hope not a plumber's fitting - never get conductors through that. As long as it is an electrical 90º you should be OK.
First I can't add too terrible much to Johns post. I will just take some time to include a little information that may help.
60 amps is plenty of power for your needs. #6 copper is great but your equipment ground wire only needs to be a #10 for 60 amps. So that would be three #6's and a #10 ground all insulated of course. You also didn't mention the distance to your garage/studio but barring anything over 130 feet or so from your power source that will suffice. As John mentioned those two 90s will make the pull a little tougher. I wouldn't try anything over #6's with #10 grd. I wouldn't hesitate to use some wire lubricant either.
On a technical note THHN is not rated for underground conduit however THWN is....that W in the rating is critical for wet location as in underground conduit runs. But your in luck as most wire now days rated THHN is also rated THWN just a bit of information so you would know. Might check but you should be fine.
As for load centers a 60 amp main breaker 12/24 load center I don't think is possible, usually these are 100 amp main breaker. But I am assuming a 60 amp breaker will be installed in your house main service rated panel to supply the sub-panel and protect your #6 copper feeder. So I would just get the 100 amp main breaker panel and let the 100 amp breaker serve as your disconnect for your detached garage/studio. The 60 amp breaker ahead of the feeder will be your OCPD protection.
As for mlo panels these simply mean lugs only are installed in the panel for your feeder conductors. Now most MLO can be configured with a backfed main breaker. Where you would install a double pole breaker usually in the first left hand 2 spaces of a panel. You then land your feeder conductors on the breaker instead of the lugs. You also must use a hold down kit on that breaker as it is a 'Main'. You then label the index on the panel cover with the included 'Main' sticker that comes with a mlo panel. In residential (not a farm) generally you can only use the mlo panel without a main breaker if the sub-panel is in the attached structure with the main service rated panel.
So in a nutshell if the mlo panel is in another structure it must have a backfed main breaker as a single throw disconnect that deenergizes all the panels branch circuits.
Most mlo panels will have a statement in the listing instructions on the panel cover that say to be suitable as service equipment a MAIN must be installed. This is to comply with NEC 225.36 where disconnects for detached buildings must be suitable for service equipment.
For your reference I'll include a diagram of a 4 wire feeder to detached structure/non-dwelling. Also a few links to some very good sites for DIY to use in their projects.
Thanks for your replies. I appreciate your taking the time to provide insight to my project.
Here's a few clarifications.
On the PVC pipe, I think I did use standard plumbing 90s. Initially I was going to have the electricians who moved our service and upgraded our main panel last year do this garage work too. They told me to bury the PVC pipe for the conduit, but didn't specify using larger radius turns. Eventually, given that this is new construction and what, too me, seems like a relatively straight-forward job that I will have inspected, I decided to save $2,000 and tackle the job myself.
There are two 90 bends in the PVC conduit run. It might be possible for me to dig up the end near the house and replace that with a larger radius turn. The one under the footing of the garage would be a b*tch to get to at this point. I do have a string coming out both ends of the conduit, so I'm hoping I can pull the string and push the wiring. I'm wondering if I should replace the string with something more robust before trying to pull the wire through. Using a smaller #10 gauge for the ground will be helpful.
On THHN vs. THWN, I couldn't determine from my research whether I had to use a wet-compatible conductor like THWN when running it through a sealed underground PVC conduit. I'll look for THWN.
The concrete guys ran a whole length of rebar for the Ufer ground. I don't know what the "standard" rebar length is, but it looked like it was 20' or longer. A handout I downloaded from the state (Oregon) says "A grounding electrode (commonly referred to as a Ufer-ground) shall be provided by installing an uncoated #4 reinforcing steel tightly attached to the foundation reinforcing steel with a minimum 12-inch splice lap and stubbed at least 12 inches above the wall plate line (see figure 3)." Figure 3 then shows a 6" minimum stub up. Unfortunately, like an idiot, I sawed off part of that stub up for reasons I no longer even remember, so now it sticks up maybe 2" above the mud sill. Am I totally hosed now? Can a longer piece of rebar be welded to what is there? Should I just consult with my electrical inspector once I have the permits?
I'm not sure I totally understand the whole main lug vs. main circuit breaker concept. It seems like it would be most straightforward to get a box that can accommodate a main breaker at the top. The whole "backfed main breaker" approach isn't super-clear to me. There doesn't appear to be a big price difference between the two types of panels, so I'll go with the type I understand the best.
THe ditch should have been inspected as well... :(
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